Many walk-behind working machines, such as lawnmowers and other similar small powered equipment, have a self-propel system that propels or drives the working machine at a selected ground speed. In such systems, a control system is typically carried on the handle to allow the operator to engage and disengage the self-propel system and to select a desired ground speed. For example, many such control systems use a pivotable ground speed control bail on the handle of the working machine. Generally, self-propelled drive systems can be divided into two categories: single/multiple speed, and variable speed. In single/multiple speed drive systems, the ground speed is fixed by one or more gear ratios, and it can only be adjusted by selecting a different gearset (if available). In contrast, variable speed drive systems allow the operator the ability to “infinitely” adjust the ground speed of the lawn mower, such as by a slipping belt system where the belt tension is varied, a slipping clutch system where the clutch pressure is varied, a hydrostatic transmission where a swash plate angle is variable, or an electric drive system where the electric power supply is switched.
Even in such variable speed drive systems, however, the maximum operating speed is either fixed or, if variable, cumbersome to change while the working machine is being operated. Specifically, in all currently available adjustable control drive systems, the maximum speed setting is made by a mechanical lever, rotary knob, or mechanical latching device. In such configurations, an operator must remove at least one of his hands from the control handle to make any adjustments to the maximum operating speed. Accordingly, making such adjustments can result in the operator at least partially losing some degree of control over the working machine. In view of these issues, it would be desirable for a ground speed control system to allow for adjustment of the maximum speed setting of the working machine without diminishing the operator's ability to control the working machine. It would also be desirable for a ground speed control system to allow for adjustment of the rate of acceleration of the working machine to enhance the operator's ability to control the working machine. With current systems, either the acceleration is fixed, or the acceleration rate must be controlled by the user when engaging the drive system by engaging the lever more or less slowly. Acceleration that is too quick can cause damage to grass by spinning the drive wheels. Acceleration that is too slow can be frustrating to the user and viewed as poor performance. The ideal acceleration rate depends on each user's preference and the conditions in which the user is operating the working machine.